This invention relates to fasteners of the type utilized to secure coverings to cementitious substrates. More particularly, this invention concerns fasteners of the type used to anchor built-up waterproofing membranes to cementitious roof decks, which fasteners improve the wind uplift resistance of the built-up roofing membrane.
Conventional "flat" roofs or roof decks often utilize a layer of hydratable cementitious material such as Portland cement type concrete to impart structural strength and rigidity and/or thermal insulating properties in the case of lightweight aggregate-containing concretes which hydrate into relatively low-density masses. Typically a waterproof covering layer is applied to the uppermost surface of the cementitious layer and such covering layer must be secured in some fashion. Most commonly the waterproof covering is in the form of several plies of sheet material bonded together by hot asphalt, each ply being applied or "built-up" on the job site and adhered together by intermittent moppings of hot fluid asphalt. The first ply, termed the "base ply", is either adhered over its entire surface to the surface of the cementitious layer using e.g. hot asphalt, or is secured only at spaced intervals using "spot" applications of hot asphalt or mechanical fasteners force through the base ply and into the soft concrete before it has fully hydrated into a hard rigid mass. The remaining plies of the built-up roofing are then secured to the base ply over their entire surfaces by hot moppings of fluid asphalts.
The mechanical fasteners or "nails" used to secure the base ply in various "spots" generally have a penetrating shank designed to resist withdrawal from the concrete layer, and a flat head which is wider in diameter than the shank thereby acting as a washer to hold the built roofing. The shank is typically made resistant to withdrawal from the concrete by causing the shank to expand in diameter near its penetrating end either as it enters the soft concrete or soon thereafter. In the fasteners for example of U.S. Pat. Nos. 3,466,967; 3,710,672 and 4,031,802 to Hallock, the shank portion of the fastener comprises a slit cone or a pair of hinged legs and is caused to expand as the fastener is inserted into the soft cementitious mass. The shank portions of these fasteners moreover define a hollow space which receives a portion of the soft cement mixture as the fastener is driven thereby increasing the resistance of the fastener to withdrawal particularly after the cement has hardened.
The nail in particular of the type shown in U.S. Pat. No. 3,710,672 and 4,031,802 Hallock has proven in practice to be especially effective for locking built-up roofing to lightweight low density expanded vermiculite or perlite containing concretes. The nail is economically formed as a one piece integral product from sheet metal stock and has a high resistance to withdrawal from the concrete layer. Difficulity has been experienced however in passing certain stringent wind uplift resistance requirements with built-up roofings fastened to such concrete layers using nails of this type. A need exists therefore for a practical manner of enhancing the wind uplift resistance of roofings secured with this and other similar types of fasteners.